In the magnetic ink inscription of documents, the documents are vacuum held on a transport belt which carries them through the inscriber station. Each individual document is sensed and then decelerated for inscribing at the inscriber station where hammers impact through a ribbon at slow speed. After inscribing, the document is accelerated and ejected at high speed. In a transport system of this type, the load range of the drive motor is highly variable depending on the vacuum gripping of the document, belt tension, frictional surfaces, etc. During the inscribing action, it is essential to maintain a constant velocity to insure accuracy of inscribing.
In prior art digital velocity servos for DC motors, such as is shown in the Agin U.S. Pat. No. 3,634,745, the systems are fixed gain servos for a given velocity error. For a given torque change, an error signal has to be generated which means the steady state velocity is in error by an amount dependent on the gain. In servos of analog type, as well as the analog-digital type, there is an upper boundary on the gain. Therefore, there is a lower boundary on velocity tolerance. Upper gain limit is determined by system stability at light load and if a heavy load is applied, velocity must shift an amount determined by the gain and this would be insufficient for the above described inscriber application. These systems were found to have restricted gain limitations, excessive drift and undesirable production set-up and field maintenance.